Today's zero emission cars in most countries just move emissions from the car to the power station (but there might be some favourable points because the energy efficiency of a steam turbine is better than that of an Otto engine).

Besides, the carbon footprint of today's batteries is not neutral, and neither is the carbon footprint of hybrid cars with their dual drive systems and batteries.

But carbon neutrality cannot be obtained in one big move. It must be a multistep process with quite a number of false tracks and failures together with the multiple small successes. And there must be buyers of the early-generation technologies so that there is finance for further development. (6502-based computers were helpful in financing the subsequent steps, but nobody would dream of using a Commodore PET for serious work these days. I drafted articles on it. It put all text into one string, so a inserting text into a long paragraph was a slow and boring process. And then came CP/M 86/80 with WordStar.)

Whatever is said about economics: There is no doubt that the local pollution is less. Specially in 3rd world megacities that would be an advantage.

Sustainable renewable - the first condition to be met is high yield. Using plants for solar energy retention is ineffective - just a few percent yield - and there is high consumption of fertilizer and much transportation involved - all with a carbon footprint. And the essential phosphorus is available in limited supply. And then there is cadmium pollution.
Or maybe not.
http://www.sciencedirect.com/science/article/pii/S1877705814011059

So we need high-yield solar energy retention. So far industry has concentrated on photovoltaics. The problem is that peak sunshine and peak demand come at different times - so there must be storage.

But you cannot run a passenger aircraft on batteries. You need a liquid carbon-based fuel with higher energy density than methanol or ethanol. You need a hydrocarbon. So we need factories that can convert CO2 photochemically into hydrocarbon fuels. Jet fuel, rocket fuel: give me pinene.

Bacteria, on the other hand, are too slow and wasteful compared to arrays of inorganic catalysts. A yield of milligrams per liter per 10 days is not enough to run a fleet of passenger jets. Or heavy lorries.

Fusion energy? The Register had an article today about alleged news - 100 MW from the size of a jet engine. Hah - who can handle 100 MW for energy purposes from such a small volume? And how about the materials surrounding the hot core - how long will they stay intact with intensive neutron irradiation?

Solar energy can be tapped today. But we need far more sophisticated systems.

there was a calculation, that all the waste+emissions caused by 10year driving a car, is still 10x less than the waste+emissions caused by manifacturing it at first place.
no wonder everyone is trying to decrease the first... and then make more, quicker-breaking cars.
macchiaveli principles, applied..

Running steel production with renewable energy is possible. We used to run iron production from ore with electricity, but it was an extremely lossy business. Even with electrical heating the carbon footprint is high. Fe2O3 or Fe3O4 + C -> Fe + CO2 remains the process regardless of the heating process.

I believe composites, when the technology is more mature, might decrease significantly the carbon footprint of car production.